This invention relates to size-selective separation membranes, and to methods of manufacturing and using them.
Size-selective separation membranes selectively retain material from a feed passing through the membrane under the influence of a pressure gradient, for example, in microfiltration, ultrafiltration, or reverse osmosis. As a rule, separation membranes are produced from rigid polymers or other rigid materials to withstand the pressure driving the filtration process without compaction, and thereby to ensure that the membrane pore structure remains essentially unchanged under normal operating pressures. The flow-rate versus pressure curves using pure fluids such as water, air etc., for such membranes are essentially linear and proportional to the applied pressure, and they are inversely proportional to the controlling thickness of the membrane.
The membrane permeability values are usually rather small for membranes used for ultrafiltration and reverse-osmosis, because the pore-sizes are rather small. To achieve practical flow rates with small pore sizes, these membranes are usually asymmetric in their structure; i.e., they are composed of a thin rigid skin which has small pores controlling the flow and separation (usually &lt;1 micron thick), supported on a porous rigid substructure. Even in this case however, the flow rate is proportional to pressure difference. The above membranes are asymmetric in terms of their morphology, but the flow-rates of the pure fluids are essentially the same and proportional to the pressure difference, irrespective of the direction of fluid flow.
Such asymmetric membranes are used with their skin sides facing the feed solution, primarily because effective "stirring" adjacent to the membrane surface is impractical in the reverse configuration. Specifically, if the feed were introduced to the more porous support side of an asymmetric membrane, it would be difficult to keep the feed well-mixed adjacent to the controlling skin, causing stagnant zones near the membrane's controlling surface and eventual build-up of high concentrations of the rejected species. This phenomenon, commonly known as concentration polarization, leads to a drastic decline in the ability of such membranes to pass fluids and retain molecules and thereby its ability to perform the intended functions is severly affected.
Concentration polarization remains a critical problem in separating large polymer molecules from a solution by ultrafiltration and, to a lesser extent, by microfiltration. The retained (rejected) solute becomes concentrated on or close to the membrane surface, and forms a gel layer which controls the transport rate. Accordingly, there have been pratical limits to the gain from improved membrane performance. Increasing the pressure only increases the thickness of the gel layer without significant increase in the flux.
Concentration polarization is currently controlled by flow management of feed-stock, which complicates the separation process and exposes solute to high shear stresses yielding a rather small actual gain in performance. Even with flow management techniques, practical fluxes are lower than the theoretical maximum, and feed-stock as well as output concentrations are limited. In addition, the energy requirements are increased. Flow management techniques are also undesirable because for many products (such as proteins) that are sensitive to shear stress.
Practical separation processes require, as a rule, several membranes of various porosities, e.g. a microfiltration membrane is used for separation of cells and other large particles from dissolved components of the fermentation broth, and subsequently an ultrafiltration membrane is used for recovering the target protein from water, salts, etc. Separate membrane modules are expensive, and the number of process components complicates the process and increases production cost.
The size-selective membranes used in microfiltration, ultrafiltration and reverse osmosis (including asymmetric membranes) generally consist of materials which are relatively rigid for the reasons given above. Examples of such rigid materials are cellulose esters, polysulfones, polyamides, polyacrylonitriles, polypropylene, polycarbonates, polytetrafluoroethylene, and derivatives of such polymers. Such membranes are available commercially from various manufacturers and suppliers, e.g. Millipore Corp. (Bedford, Mass.); Gelman Sciences Ann Arbor, Mich.; Amicon Corp. (Danvers, Mass.). The membranes are commonly prepared by phase-inversion processes, the principles of which are well-known to those in the art (see R. E. Kesting, Synthetic Polymeric Membranes, A Structural Perspective, 2d Ed., Wiley-Interscience, NY, N.Y.; 1985 Ch. 7, pp. 237-238.) In general, such membranes are asymmetric as to pore size, but not as to flow rate--i.e., the flow rate of pure fluid is essentially the same regardless of whether the high pressure is applied to the denser side or to the more porous side.
For example, one material that has been used for reverse osmosis membranes is cellulose acetate. Such membranes are intended to be rigid, although, as with many plastics, they may experience undesirable creep. The high-pressure side of the membrane consists of a relatively dense skin containing pores small enough to control flow. When very high pressures are applied to the opposite side of such membranes, it has been observed that the pores in the skin enlarge, allowing greater water permeation and lower salt rejection than in the normal orientation (high pressure applied to the skin side). [See, Sourirajan and Matsura, Reverse Osmosis/Ultrafiltration, Canadian National Resourse Council (1985); and Banks and Sharples 1966 J. Appl. Chem. 16:28-32.]While the pore size enlargement in some situations is temporary, Kopeck and Sourirajan (1969) J. Appl. Polymer Sci. 13:637-659 propose that if very high back pressure is repeatedly applied for prolonged periods during the membrane manufacturing process, creep recovery will be incomplete and membrane pore shape will be improved. In this way, the authors suggest, product recovery will be increased without significant decrease in the membrane's ability to reject salt. Back pressure is used as part of pore shaping only during manufacture. In this pore-shaping process, the anisotropic difference in flow rates generally is below 10 in the first cycle, and is usually lower. The pressures used tend to be very high (over 600 psi). After about five cycles, the pores are permanently expanded, and the shaping process is complete. The authors also note that, over time, cellulose acetate membranes operated in the standard configuration experience undesirable compaction, apparently due to creep.
There have been other reports of asymmetric transport. Rogers et al. (1957) Industrial and Engineering Chem. 49(11):1933-1936 disclose a composite membrane in which the rate of water vapor permeation depends on the direction of flow. The membrane is a composite of Nylon 6 and Ethocell 610 (plasticized ethylcellulose). Such membranes, however, are not size-selective "pore-type" membranes, in the sense that their anisotropic behavior is a function of differential solubility of the permeating substance at the two membrane surfaces. The flow anisotropy with such membranes is generally small.
Nichols U.S. Pat. No. 3,846,404 discloses one specific cellulose triacetate (CTA) gel product formed from a solution by solvent replacement. CTA is dissolved in a solvent; then a gellant (i.e., a liquid that is miscible with the solvent, but is not a CTA solvent) is added. Once dry, the product is not reswollen by water. In film or capillary fiber form the product is said to be useful for various purposes including as a membrane.